As memory devices fabricated in silicon and other semiconductor materials have become increasingly complex, it has become common practice to include in each device a certain number of redundant elements. These elements are activated and defective elements are de-activated, typically through the use of on-circuit fuses, when the inevitable manufacturing defects appear in the primary elements.
The storage elements of such memory devices are arranged in a matrix of columns and rows, so that an individual element may be addressed by selecting the appropriate column and row. It has become common to include entire redundant rows and columns on each memory device and to include additional circuits to replace a defective column or row with one of the redundant columns or rows.
In some prior art memory devices with redundancy, fuses associated with each storage element are "blown" by means of a high current or a laser beam to replace a defective element with a redundant one. This, of course, requires substantial die area for the large number of required fuses.
Other types of prior art memory devices with redundancy are capable of replacing entire columns of storage elements with redundant ones. This is accomplished by including a redundant address decoder, which can be programmed by fuse-blowing, on the die. The redundant decoder responds to those column addresses corresponding to defective columns by providing an electrical deselect signal to the normal decoder and providing a redundant select signal which acts to allow a redundant column access to an I/O line normally used by one of the normal columns.
This later scheme for redundancy is disadvantageous when access time is critical. The time necessary to allow the redundant column decoder to evaluate the address, inhibit the normal column decoder and select the redundant column is significant when compared to normal access time. Thus, parts using redundant columns are slower than parts in which none of the redundant columns are used.
It is possible to avoid the problem of increased column access time by including a fuse at the sense amplifier of each column and blowing the fuses of defective normal columns, but this approach leaves "floating" normal storage elements.